How Tall Is That Tree - Pencil Method

This one comes from the 1948 edition of the Boy Scout Handbook. (A great find last week at an antique mall for only $6.00!)

Pencil Method - Have a friend whose height you know stand against the tree (your walking stick will also work if you know the length of it). Hold a pencil or short stick at arms length, sight across the top of it to the top of your friend's head. At the same time move over your thumb on the stick until you sight across it to the base of the tree.  Then raise your arm and the stick, until your line of sight over your thumb hits the top of your friends head. Note where your line of vision across the top of the stick cuts the tree.  Move your arm up again, and repeat the process.  Thus you find out the number of many times higher the tree is, than the known height (your friend or walking stick).  Multiply that number by the known height, to find the height of the tree.